1. Field of the Invention
The present invention relates to an exterior heat insulation panel which uses vaporization heat of water and is used for cooling and air-conditioning of structures, particularly, to an exterior heat insulation panel which can achieve high efficiency and energy-saving.
2. Description of Related Art
Recently, global warming and urban heat islands due to excessive use of fossil fuels have become urgent issues to be resolved. In this context, in order to deal with the issue, various new energies and energy saving systems have been proposed so far.
The roof, walls, and the like of a building readily reach a high temperature during daytime since they are exposed to direct sunlight for a long time. Particularly, during the summer when solar radiation hours are long, the heat absorbed from the surface of a roof and walls is transferred to inside the structure, thereby elevating the room temperature. The heat transferred from the surface of the roof and walls to a structure is accumulated, thereby decreasing air-conditioning efficiency. Currently, among widely spread energy-saving systems of buildings, an exterior heat insulation method can be exemplified as one of the most efficient methods.
The exterior heat insulation method is a method of providing heat insulation layers outside a building. According to this method, structures having a large heat capacity, such as a concrete structure, are protected from outside heat and cold; therefore, it is possible to suppress a large rise in temperature due to solar radiation heat during daytime of the summer or a drop in temperature due to the accumulation of cold air at night time in winter in buildings. Meanwhile, the interior temperature of a building is easily maintained by the large heat capacity of the building itself due to the effect of exterior heat insulation. Accordingly, compared to the conventional interior heat insulation, that is, the method of providing heat insulation layers inside a building, it is possible to drastically reduce the energy required for air-conditioning and obtain an advantage that room temperature can be easily maintained to make a pleasant environment.
The exterior heat insulation method has been limited to costly construction methods because of the needs of fire resistance of exterior walls by now, thus, exterior heat-insulated buildings have been spread slowly. However, 30 years or more of performance results in Japan, Europe, and America, introduction of inexpensive exterior heat insulation methods with confirmed fire resistance performances, and measures for energy-saving effect based on the Kyoto Protocol are providing impetus for the recent increase in the new construction and renovation of exterior heat-insulated buildings.
As a general method of the exterior heat insulation method, heat-retaining boards such as foamed polyethylene and foamed polystyrene are provided on the surface of a roof or walls, and surface layers such as cinder concrete and pressing blocks are provided outside thereof for the purpose of prevention of ultraviolet degradation of the heat-retaining boards, thereby achieving improvement of fire resistance, and antiscattering, in order to protect the heat-retaining boards. However, such a method only has a heat-insulating effect of inhibiting heat-transfer by heat transmission resistance of the heat-retaining boards and is not able to actively cool the surface of a roof. Also, the surface layer reaches 65° C. in the summer, and the heat accumulated in the surface layer is leaked through the heat-retaining board and transferred to the building structure and then accumulated therein. The heat accumulated in this manner is gradually transferred inside the structure even when there is no solar radiation such as at night time or during cloudy weather. In other words, this method is not effective as a fundamental measure of energy-saving in air-conditioning and improving habitability. In addition, the method has a problem in that it encourages to form heat islands due to heat radiation from the surface layer at high temperature.
As a method of directly removing heat from a structure, there is a method of so-called “water sprinkling” using vaporization heat of water. “Water sprinkling” is one of the effective methods of actively reducing heat and has been customarily used in many places. Also, “water sprinkling” has been empirically proved to be highly effective in cooling and is drawing attention recently as means for achieving reduction of energy in air-conditioning by cooling a building.
However, when sprinkled or supplied to the surface of a roof or walls of a structure, water immediately flows down due to the slope of the structure. Moreover, the cooling effect is obtained only at the site of the structure to which the water contacts. That is, in order to stably cool the entire surface of a roof and walls evenly and for a long period of time, it is necessary for building materials to have a water retentive function. Accordingly, there have been various suggestions regarding such water retentive building materials. For instance, as the most frequently found example, there is a pavement with water-permeability and water retentivity. The pavement with water-permeability and water retentivity is drawing attention as a countermeasure, which uses rainwater effectively, against urban heat islands. Regarding the water retentive building material, several inventions and devices are also disclosed in addition to the example.
For example, Japanese Unexamined Application Publication No. 2006-283447 discloses an invention under the title of “Water retentive pavement structure”, the invention relating to a pavement structure having water retentivity which can reduce road surface temperature using vaporization heat of water.
The invention disclosed in Japanese Unexamined Application Publication No. 2006-283447 is characterized in that rainwater is stored on a water impermeable layer, an aggregate layer mixed with a water retentive material is provided on the water impermeable layer, a water absorbing material is laid on the aggregate layer, and a water retentive block and a water permeable block are intermingled and arranged on the water absorbing material.
In this configuration, the rainwater, which flows into the aggregate layer mainly by permeating through the water permeable block, evaporates through voids of the aggregate layer due to temperature elevation of road surface, and thus generated vaporization heat yields an action of suppressing temperature elevation of the road surface.
Also, Japanese Unexamined Application Publication No. 2005-145771 discloses an invention under the title of “Water permeable block”, the invention relating to a block which improves a water permeable function, a water retentive function, and a water purifying function and which can achieve limestone recycling.
The invention disclosed in Japanese Unexamined Application Publication No. 2005-145771 is characterized in that cement is mixed with coal and coal ash and then formed into a block shape.
A block in this configuration has a high water permeable function, a water retentive function, and a water purifying function, and therefore it is possible to use the block as a water retaining and evaporative cooling material for counteracting against urban heat island. It is also possible to recycle the fly ash which has been subjected to the waste disposal.
However, for the water retentive building material used in the surface of a roof and walls, conflicting performances such as water retentivity, strength, freeze-thaw resistance, and weight are required. Generally, it is very difficult to balance these performances. For example, in order to improve a water retention rate of water retentive building materials (an amount (volume) of water retained with respect to volume of building material), small voids are formed in the building material or a porous aggregate having high void ratio is used; however, in this case, the compression tensile strength, and bending strength of water retentive building materials, significantly deteriorate. Particularly, strength against impact load is significantly deteriorated, thereby causing difficulty even in light walking. Therefore, sites to which the water retentive building material can be applied are greatly limited. Generally, when the ratio of void volume to the building material volume is the same, it is confirmed that there is a tendency that the smaller the void, the higher the strength. However, when water in the void is frozen and expanded in the winter, the water retentive building material is cracked or broken. There is a method of thickening the water retentive material by reducing the water retention rate of water retentive building materials, but in this case, while strength and water retentivity can be secured, weight of the water retentive building material itself increases. Many existing buildings have low load resistance, so it is difficult for the above method to be applied to the existing buildings accounting for more than half of structures.
As described so far, though confirmed to be effective to some degree, the existing water retentive building material disclosed in the patent publications noted above is far from practical for use.